This invention relates generally to platens for positioning recording mediums and more particularly to an evacuated platen for accurately positioning the medium during recording at a printing station, then guiding the medium during advancement.
In certain recording applications, such as nonimpact printing with ink droplets, the positioning of a paper web becomes critical. Slight lateral motion of the web or variation in the increment of advancement produce noticeable faults in the printing. The permitted positional tolerance is frequently on the order of 0.1 mm at the printing station.
The use of vacuum tensioning devices is well known in the art of controlling paper in web form. Examples of vacuum tensioning devices are disclosed in U.S. Pat. Nos. 2,710,154 and 3,259,288. These devices are used to effect a drag on the web. In the first patent, the guiding means attempts to maintain a constant tension on the web; in the second patent, the vacuum station effects a drag and reserve paper loop which is necessary to obviate the need for phased pairs of drive tractors. The second patent will also prevent the web inertia from causing overshooting at a subsequent printing station when the advancing means suddenly stops after a line spacing operation. These devices, however, do not provide a suitable printing station at the vacuum device and cannot accurately limit the lateral motion of the web during its advance.
A printing station in general requires that the web be maintained flat at the recording area without lateral deviation during advancement. One technique of insuring web alignment is to place the web under tension over a plurality of ridges that are parallel to the motion of the web. An example of such a guiding device is shown in U.S. Pat. No. 3,549,068. The guiding surface of this device, however, is curved and thus unsatisfactory for holding the paper web during printing.